Interesting article.
http://www.theguardian.com/science/2...-missing-stars
In summary, simulations seem to wildly over-estimate star formation. The solution seems to be that radiation pressure from already-formed stars in a galaxy blows away the remaining gas from the galaxy, halting star formation.
I like this as a wonderfully neat and comprehensible explanation, which seems obvious once pointed out.
Of course just because something obvious doesn't necessarily mean it's true, hence all the observation and testing.